1. Field of the Invention
The present invention relates to a liquid-crystal display device, and more particularly to a liquid-crystal dependent, optical writing, liquid-crystal display and recording apparatus that may be used with recording systems, image display systems, image storage systems and any other systems that adopt reversible recording media. The present invention also relates to a method and apparatus for the display of liquid-crystal images that are capable of displaying fast moving images and which can still hold the displayed image without application of an electric field.
2. Discussion of the Related Art
Unexamined Published Japanese Patent Application (kokai) Nos. Hei 4-94281, 4-130420, 4-136821 and 4-218021 teach display devices that have a photoconductive layer and a light-controlling layer formed in superposition and in which, with an electric field being applied to both layers, the photoconductive layer is exposed on the basis of image information whereas the alignment of the liquid crystal in the light-controlling layer is controlled to write an image.
Let us describe individually the technical features of the inventions proposed by these patent applications. The invention proposed by Unexamined Published Japanese Patent Application (kokai) No. Hei 4-94281 aims at producing an image of high resolution at a low manufacturing cost; the invention proposed by Unexamined Published Japanese Patent Application (kokai) No. Hei 4-136821 has as a principal object the production of a high-resolution image with a multilayered structure of a dielectric mirror; the invention proposed by Unexamined Published Japanese Patent Application (kokai) No. Hei 4-218021 is such that a pulsed voltage of opposite polarity but of the same magnitude as in the writing mode is applied and, in synchronism with the applied voltage, a laser beam is emitted under completely identical conditions to those used in the writing mode, whereby the written information is partly erased leaving the necessary information intact.
With a view to improving the user interface, a demand has arisen recently to develop electronic devices that are oriented for friendliness to the human eye and greater ease in handling. In particular, electronic display devices are mostly used in operations for the primary purpose of drafting or displaying documents, and hence, it is desired to develop a display device that is capable of eye-friendly passive display, that has high reflective contrast and which satisfies two conditions, one for faster display and the other for the storage of display. Several display technologies have been proposed to date that satisfy part of these requirements. One known example of such technologies is a display device that makes use of the light scattering mode of a liquid crystal. In such a display device that operates in the light scattering mode, a liquid crystal is mixed with a transparent material having a substantially equal refractive index to the liquid crystal when it is aligned vertically and in order to align the liquid crystal either vertically or randomly, heat or an electric field is applied for controlling its alignment. In a display like this, light is transmitted when the liquid crystal is vertically aligned whereas in random alignment, light is scattered on account of the index difference and fails to be transmitted.
A materials technology to be used in display devices operating in the light scattering mode is taught by Japanese Patent Domestic Announcement (kohyo) No. Sho 58-501631 and U.S. Pat. No. 4,435,047; according to these references, an encapsulated liquid crystal is dispersed as crystal globules in a polymer, which is then processed into a film. According to the disclosed technology, the liquid crystal confined in capsules typically made of a polymer exhibits the nematic phase of positive dielectric anisotropy at the temperature of the environment of use; if the liquid crystal is placed in an electric field, the vector of its alignment becomes parallel to the field direction and the refractive index of the liquid crystal becomes equal to that of the polymer, whereupon the film turns transparent; if the electric field is removed, the molecules of the liquid crystal are aligned randomly and incident light is scattered at the interface between the liquid crystal and the capsule wall, whereby light transmission is blocked to produce turbid areas in the film for displaying the recorded information.
Japanese Patent Domestic Announcement (kohyo) No. Sho 63-501512 and Unexamined Published Japanese Patent Application (kokai) No. Sho 63-155022 propose that a liquid crystal exhibiting the smectic phase at an ordinary temperature be contained in a film to provide it with memory quality. If such materials are used, the written image can be held without power supply on account of the memory quality of the liquid crystal, and furthermore, the recording medium can be processed into a film, thereby making it possible to handle the display screen as if it were a hard copy.
An exemplary method of writing image with a display device using the above-described liquid-crystal materials is taught in Unexamined Published Japanese Patent Application (kokai) No. Sho 63-155022; according to this method, a voltage is applied to a binder layer containing microcapsules confining a smectic liquid crystal and the layer is heated with a thermal head that is driven in accordance with image information, whereupon the liquid crystal's alignment varied to control light in such a way that it is transmitted or not transmitted.
Unexamined Published Japanese Patent Application (kokai) No. Hei 3-155525 teaches an information recording medium that is composed of a photoconductive amorphous silicon layer in combination with a polymer-liquid crystal composite film that has a liquid crystal exhibiting the smectic phase at a room temperature dispersed in a polymer matrix; in this medium, an optical image is focused on the photoconductive layer and the polymer-liquid crystal composite film is provided with a field intensity distribution in accordance with the exposing pattern.
Further, Unexamined Published Japanese Patent Application (kokai) No. Hei 4-86618 teaches that a recording medium comprising a substrate that is deposited with a polymer-liquid crystal composite film having a liquid crystal dispersed in a polymeric material can provide continuous-tone display by controlling the intensity or area of heating.
The above-mentioned Unexamined Published Japanese Patent Application (kokai) Nos. 4-94281, 4-130420, 4-136821, 4-218021 and 3-155525 make no disclosure of technologies on continuous-tone display.
The display devices disclosed in Unexamined Published Japanese Patent Application (kokai) Nos. Sho 63-155022 and Hei 4-86618 are incapable of fast rewriting of displays since the rewrite speed is determined by the rate of heat transfer. As a further problem, the production of continuous-tone display requires precise control of the intensity of heating; however, variations in the characteristic of heating elements present considerable difficulty in achieving correct control over a number of density levels.
Another display apparatus is proposed in Unexamined Published Japanese Patent Application (kokai) Hei 2-93432; a polymer-dispersed liquid crystal such that a liquid crystal exhibiting the nematic phase is dispersed irregularly in a polymeric material is loaded between a substrate and an opposing matrix electrode plate to fabricate a liquid-crystal display device and an electric field in accordance with image information is applied to the device via the matrix electrode, whereby the liquid crystal's alignment is controlled in such a way as to form light-transmitting or light-opaque areas, thereby effecting image display.
The liquid-crystal image display apparatus which is taught in Unexamined Published Japanese Patent Application (kokai) No. Sho 63-155022 is capable of holding the display screen since it uses a liquid crystal that exhibits the smectic phase having memory quality. However, the speed of rewriting the displayed image is limited by the heat transfer speed of the thermal head and, what is more, the thermal head or the liquid-crystal display medium has to be scanned in one direction to achieve image rewriting and, hence, image rewriting and display cannot be achieved as fast as can be done by ordinary electronic displays. Furthermore, the liquid-crystal display medium has such a structure that liquid-crystal globules having the liquid crystal of the smectic phase incorporated in microcapsules are dispersed in the polymer and, hence, an applied electric field will act upon the liquid crystal globules via the polymer and capsules. As a result, high drive voltage is required to cause a change in the arrangement of the molecules of the liquid crystal and, in addition, in order to hold the molecular arrangement of the liquid crystal in the smectic phase, the applied voltage (holding voltage) must not be lower than the level necessary to cause a change in the liquid crystal's molecular arrangement. This in turn causes a further disadvantage in that there is a potential hazard for failure due to current impression if a high voltage is applied to provide practical image memory quality or in that contrast is lowered if the thickness of liquid crystal layer is reduced in order to render it susceptible to the applied electric field.
The liquid-crystal image display apparatus disclosed in Unexamined Published Japanese Patent Application No. Hei 2-93432 is capable of high-speed image rewriting since image has been written by applying electric field to a liquid crystal that assumes the nematic phase at ordinary temperatures; however, the liquid crystal per se has no memory quality and, hence, the display screen cannot be held.
To impart memory quality to this type of liquid-crystal display device, either one of the following methods may be employed. In the first method, a thermoplastic resin is used as the polymer and its capability for alignment is lost by first heating to a temperature not lower than its glass transition point and, then, with voltage being applied, the polymer is cooled slowly enough to impart alignability (see Japanese Patent Domestic Announcement (kohyo) No. Sho 63-501512). In the second method, an azo dye is incorporated in such a way that it is present at the interface between the polymer and the liquid crystal and cis-trans isomerization is controlled by exposure to visible light or uv radiation (see Onoki, Hayashi, Mizushima and Yamamoto, "Controlling the Formation of Liquid Crystal in Mixed Liquid Crystal-Polymer System by Optical Isomerization of Azobenzene Structure" in the Journal of the Chemical Society of Japan, No. 8, pp. 815-818, 1990). However, this method of controlling the liquid crystal's alignment with a view to imparting memory quality is very cumbersome. Yet, the result of this cumbersome operation is no more than simple memory quality.
Thus, there has been available no image display technology that has a dual characteristic in that image rewriting can be done at high speed and that the once written image can readily be retained. Under the circumstances, it has been desired to develop a liquid-crystal image display technology that is satisfactorily applicable to moving images.